H2 O2 as a conditioning agent for electrostatic precipitators

ABSTRACT

A method of improving the efficiency of electrostatic precipitators for removing high resistivity particulate matter from flue gases by treating said flue gases prior to contact with the electrostatic precipitator with an aqueous solution of hydrogen peroxide with the ratio of hydrogen peroxide to SO 3  being on a weight basis of at least 0.5/l.

INTRODUCTION

One of the problems relating to the collection efficiency ofelectrostatic precipitators, ESPs, is the high particulate resistivitywhen the boiler burns low sulfur western coal. High fly ash resistivityaffects an ESP efficiency principally by limiting the voltage andcurrent at which the precipitator operates. (See L. A. Midkiff, Flue-gasConditioning Upgrades Performance, Cuts Down Size of Precipitators,Power, April 1979, p. 99.) Commercial conditioning agents are sulfurtrioxide (SO₃), sulfuric acid (H₂ SO₄), ammonia, ammonium sulfate, etc.They are injected into the flue gas in the form of either a fine powderor an aqueous solution.

The present invention relates to the discovery that with a small dosageof hydrogen peroxide, H₂ O₂, (10-50 ppm), the amount of SO₃ /H₂ SO₄ inthe flue gas can be increased effectively by oxidizing the existingsulfurdioxide (SO₂) to SO₃ /H₂ SO₄ without introducing additional sulfursources.

THE INVENTION

A method of improving the efficiency of electrostatic precipitators forremoving high resistivity particulate matter from flue gases by treatingsaid flue gases prior to contact with the electrostatic precipitatorwith an aqueous solution of hydrogen peroxide (H₂ O₂) with the ratio ofH₂ O₂ to SO₃ being on a weight basis of at least 0.5/1.

The concentration of the H₂ O₂ can vary between 2 percent by weight upto its solubility in water. A preferred concentration is between 15 and30 percent by weight.

The amount of H₂ O₂ used to treat the SO₃ in the flue gas may be aslittle as 0.5 parts per weight to 1 part per weight of SO₃. The dosagemay be varied to provide a weight ratio of H₂ O₂ to SO₃ of from 0.5/1 to2/1.

The peroxide effectively and efficiently converts the SO₂ to SO₃ whenthe flue gas is at a temperature of about 300° to 400° F.

The overall reactions of H₂ O₂ can be simplified to 5 general types asfollows*:

    ______________________________________                                        Decompositon  2 H.sub.2 O.sub.2 → 2 H.sub.2 O + O.sub.2                Molecular Addition                                                                          H.sub.2 O.sub.2 + Y → Y.H.sub.2 O.sub.2                  Substitution  H.sub.2 O.sub.2 + RX → ROOH + HX                                       or H.sub.2 O.sub.2 + 2 RX → ROOR + 2 HX                  H.sub.2 O.sub.2 as                                                                          H.sub.2 O.sub.2 + Z → ZH.sub.2 + O.sub.2                 Reducing Agent                                                                H.sub.2 O.sub.2 as                                                                          H.sub.2 O.sub.2 + W → WO + H.sub.2 O                     Oxidizing Agent                                                               ______________________________________                                         *From Encyclopedia Chem. Tech., Vol. 11, p. 394 (1966)                   

As an oxidizing agent, peroxide reacts with the SO₂ in flue gasaccording to the reaction: H₂ O₂ +SO₂ →SO₃ +H₂ O, or it may decompose ordissociate into the oxygen atom in the flue gas before it reacts withSO₂. Stoichiometric ratio of the above reaction is 1/1 as a molar ratioor 1/1.88 as a weight ratio of H₂ O₂ /SO₂. The conversion efficiency ofSO₂ (ppm) to SO₃ (ppm) is defined as: ##EQU1##

EVALUATION OF THE INVENTION

1. Bench Mini-scrubber:

Oxidation of SO₂ to SO₃ was done with a bench scale, spray type scrubberin which SO₂ gas was mixed with fine droplets of H₂ O₂ solution. Thegeneral arrangement of the bench mini-scrubber is shown in FIG. 1. Theapparatus consists of three main parts:

Furnace

Scrubber

SO₂ analyzer.

A flow of 12 SCFH of diluted SO₂ (3000 ppm) was passed through theelectrically heated furnace which was preset at around 1500° F. The gasmixture then entered the scrubber. The scrubber is a spray type of 8"height, 11/2" inside diameter. Hydrogen peroxide solution was sprayedfrom the top of the scrubber and reacted with the incoming SO₂ to formSO₃ /H₂ SO₄. The hot gas was cooled as it passed the condenser. About 2SCFH of the exhaust gas was introduced into the SO₂ analyzer (ThermoElectron's pulsed fluorescent SO₂ analyzer). Gas temperature at thescrubber inlet was from 300° F. to 400° F.

Measurement of the baseline SO₂ concentration (in ppm) started after thesteady state condition of the system was reached. Hydrogen peroxide (30%solution) was then injected into the scrubber. The treatment dosageswere from 1/2 to 1/1 by weight ratio of H₂ O₂ /SO₂. The conversion ofSO₂ to SO₃ is defined as the percentage change of the SO₂ measuredbefore and after the chemical injection. The conversion efficiency isexpressed in equation 1.

2. Mini Combustor:

This simulation combuster was used to determine the oxidation of SO₂ byH₂ O₂ as an intermediate step between the bench scale and the processsimulation experiments. The unit can burn gas or fuel oil at the rate of10,000 to 30,000 BTU/Hr. and includes four major components (FIG. 2):

Fuel feeder

Burner

Furnace

Exhaust system.

The combustor was first warmed up with propane gas for 1/2 hour, thenswitched to fuel oil No. 2. When the combustor reached steady state, arequired concentration of SO₂ gas was then injected into the furnacechamber. H₂ O₂ solution was sprayed at the inlet of the stack gas.Measurement of the SO₂ concentration was done before and after thechemical injection to determine the conversion efficiency of SO₂ to SO₃as expressed in equation 1.

3. Pilot Electrostatic Precipitator (ESP):

The pilot ESP, shown schematically in FIG. 3, was designed for thepurpose of testing candidate fly ash conditioning agents. The basiccomponents include:

Burner

Flue gas system

Fly ash feeder

Chemical feeder

SO₂ injector

Control panel

ESP unit.

The unit incorporates flue gas derived from a 350,000 BTU/Hr. oilburner. The tested fly ash is metered by a modified 9H miniveyor and fedinto the flue gas duct at 700° F. The ESP unit is located 20 ft.downstream from the burner. It is rated at 100 SCFH and has a collectorplate area of 48 ft.². The control panel features a milliamp-meter,kilovolt-meter, spark rate meter and power stat. Since the fuel oil usedwas low in sulfur content (0.2%S), injection of SO₂ gas was necessary toraise the SO₂ level in the flue gas to 2500 ppm.

Chemical additive, as H₂ O₂ solution, was sprayed into the flue gasduct. An air blast nozzle was used to disperse the fine droplets of H₂O₂ into the gas stream. Chemical feed rate was calibrated by volume flowrate. Measurement of major species such as O₂, CO₂, CO, NO_(x), and SO₂was done continuously at the ESP inlet. Flue gas velocity in the testsection was from 15 to 25 ft/sec. and the gas temperature could beadjusted from 300° F. to 500° F. Measurement of SO₂ concentrations wasdone with the Thermo Electron's SO₂ analyzer. The conversion efficiencyis expressed in equation 1.

The results can be summarized as follows:

    ______________________________________                                        Test Equipment                                                                            Additive SO.sub.2 (initial)                                                                      SO.sub.2 (final)                                                                      E %                                    ______________________________________                                        Bench-miniscrubber                                                                        V.sub.2 O.sub.5                                                                        2650      250     91                                                 Al.sub.2 O.sub.3                                                                       3100      2900     6                                                 MnO      2600      500     81                                                 Na.sub.2 SO.sub.4                                                                      3050      3350    -10                                                Fe.sub.2 O.sub.3                                                                       2050      1150    44                                                 H.sub.2 O.sub.2                                                                        2640      280     89                                     Minicombustor                                                                             H.sub.2 O.sub.2                                                                        1900      200     89                                                 Water    2100      1900    10                                     ESP         H.sub.2 O.sub.2                                                                        2750      250     91                                                 H.sub.2 O.sub.2                                                                        2500      250     90                                                 H.sub.2 O.sub.2                                                                        1930      230     88                                     ______________________________________                                    

During the last run on ESP, changes in the secondary current of the rearsection were observed. They were as follows:

    ______________________________________                                        Initial condition 105-109 milliamp                                            (with SO.sub.2 injection)                                                     Final condition   148-150 milliamp                                            (with SO.sub.2 and                                                            H.sub.2 O.sub.2 injection)                                                    with H.sub.2 O.sub.2 injection only:                                                            125-130 milliamp                                            ______________________________________                                    

The results indicated an increasing of SO₃ /H₂ SO₄ concentration insidethe ESP in the presence of H₂ O₂.

Having thus described my invention, it is claimed as follows:
 1. Amethod of improving the efficiency of electrostatic precipitators forremoving high resistivity particulate matter from flue gases by treatingsaid flue gases prior to contact with the electrostatic precipitatorwith an aqueous solution of hydrogen peroxide with the ratio of hydrogenperoxide to SO₃ being on a weight basis of at least 0.5/1.